This invention relates to injection molding and more particularly to an improved bushing to be positioned around each valve pin to support the mold back plate in a multi-cavity valve gated injection molding system.
In such a system wherein a melt passage branches in a manifold and extends through a heated nozzle to each gate, it is necessary to provide an insulative air space between the hot manifold and the cooled mold back plate. A seal must be provided to prevent leakage of pressured melt around the reciprocating valve pin into the air space between them. The manifold must also be retained tightly against the nozzles to prevent melt leakage between them.
In the past, it has been known to use spacers seated in the mold back plate to space the manifold from the mold back plate and provide the insulative air space. Alternatively, bolts extending from the manifold into the cavity plate can be used to retain the manifold against the nozzle to prevent leakage between them and to properly locate the manifold to provide the necessary air spaces. An example of this structure is shown in the applicant's U.S. Pat. No. 4,530,654 which issued July 23, 1985.
It is also well known to provide a bushing seal with a central bore through which the valve pin fits snugly to prevent leakage of melt into the air space. One example in which the bushing seal seats in the nozzle is shown in U.S. Pat. No. 4,026,518 to Gellert entitled "Bushing Seal for Valve Gated Injection Mold" which issued May 31, 1977. Another example in which the bushing is fixed to the nozzle is shown in U.S. Pat. No. 4,433,969 to Gellert entitled "Injection Molding Valve Pin Bushing and Method" which issued Feb. 28, 1984.
However, as seen in U.S. Pat. No. 4,530,654 referred to above, these previous systems have the disadvantages that the manifold must be drilled on the diagonal bores to provide the melt passage and the bolts must be provided extending into the cavity plate to tighten the manifold against the heated nozzles. This adds to manufacturing costs and increases the complexity of molularization of manifold systems.